FIGS. 7 through 10 show an example of a prior art suspension system of a wheel type hydraulic shovel. FIG. 7 is a side view of a lower propulsion unit of the wheel type hydraulic shovel. The reference symbol 1 denotes a front wheel axle and the reference symbol 2 denotes a rear wheel axle, and a front drive shaft 1a and a rear drive shaft 2a are inserted in central cavity portions of these axles 1 and 2. Propulsive force is transmitted from a transmission 3 to these front drive shaft 1a and rear drive shaft 2a via shafts 4 and 5. As shown in FIG. 8, a chassis frame 7 is connected to a central upper portion of the axle 1 for the front wheels via a pin 6 so as to be able to swivel in the lateral direction of the vehicle. A driver's seat and a working attachment made up from a boom, an arm and a bucket and the like, not shown in the figures, are provided on a rotating frame which is provided on the chassis frame 7 so as to be rotatable, and as shown in FIG. 8 single acting cylinders 8 are respectively fitted on the left and right ends with respect to the vehicle of the chassis frame 7. In FIG. 8, the reference symbols 11 denote tires.
In the single acting cylinders 8, as shown in FIGS. 9 and 10, a tube 8a is fixed to the chassis frame 7 by bolts 9 via a flange portion 8d, and the end of a piston rod 8b is contacted against a bracket 10 which is projected from the axle 1. During vehicle propulsion, the cylinder chambers 8c of the cylinders 8 are connected to a tank, so that piston rods 8b are freely extended and retracted, and thus the ground contacting performance of the left and right tires is improved. During excavation the cylinder chambers 8c are hydraulically locked, so that the stability of the vehicle body is assured during excavation by fixing the chassis frame 7 with respect to the axle 1.
However, with such a prior art suspension system of a work vehicle, since the axle 1 and the chassis frame 7 are directly connected together by the pin 6 so that no shock absorbing device is provided between them, and therefore during vehicle propulsion shocks from the road surface are directly input to the chassis frame, and there has been a problem with regard to riding quality. Further, although the moment load around the pin 6 caused by the rolling load during vehicle high speed cornering acts on the chassis frame 7, the single acting cylinders 8 keep free, so this moment load cannot be supported, and the chassis frame 7 rolls undesirably.
Although a suspension system used on a dump truck or the like can itself support a chassis frame of considerable weight, it is necessary for a work vehicle such as the wheel type hydraulic shovel or the like to which the present invention is addressed to perform work other than normal vehicle propulsion such as excavation or the like, and it is not possible to use a suspension system for a truck or the like, since large reaction forces to excavating are required to be supported in all directions longitudinally, laterally, and vertically to the vehicle.